Electron detector for selectively detecting secondary electrons and high-energy reflected electrons



April 1968 I-IIRosI-II OKANO 3,38 32 ELECTRON DETECTOR FOR SELECTIVELYDETECTING SECONDARY ELECTRONS AND HIGH-ENERGY REFLECTED ELECTRONS FiledFeb. 25,- 1965 Fl (3 4 F i G. 3

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United States Patent 3,381,132 ELECTRON DETECTGR EUR SELEQTWELYDETECTiNG SECONDARY ELEQTRDNS AND HlGH-ENERGY REFLEtITED ELECTRONE:Hiroshi Ghana, Hachioji-shi, Japan, assignor to Kabushihi Kaisha HitachiSeisalnzsho, Chiycda-iru, Tokyo-to, Japan, a joint-stock company ofJapan Filed Feb. 25, 1965, Ser. No. 435,131 Claims priori y, applicationJapan, Feb. 27, 1964, 39/ 10,499 3 Claims. (Cl. 250-333) ABSTRACT 6? THEDESCLOSURE An electron detector for selectively detecting secondaryelectrons and high-energy reflected electrons from a specimen subjectedto electron bombardment, which comprises an electrically conductive filmwhich detects the secondary electrons but causes the high energyreflected electron to pass therethrough, and an electron collectordisposed behind the conductive film to collect the high energy reflectedelectrons passing through the conductive film.

This invention relates to electron detectors and more particularly to anew electron detector capable of selectively detecting only secondaryelectrons and high energy reflected electrons and of operating withamply high signalto-noise ratio with respect to electrons of low energy.

Heretofore, reflected electron images formed by X-ray microanalyzers anda scanning electron microscope have been formed principally by reflectedelectrons with high energy from the specimen.

The principal reasons for this practice have been that it has not beenpossible for the detector for electrons beams to discriminate only thesecondary electrons and that, particularly in the case when ascintillation counter is used, ample signal-to-noise ratio with respectto electrons of low ener y of the order of a number of tens of electronvolts could not be obtained whereby the detection performance droppedsubstantially.

However, there is a great need for determining differences in emissivityof secondary electrons due to differences in specimen material, surfacecondition, surface electric potential, and other conditions.

It is a general object of the present invention to provide means forsatisfying this need.

More specifically, it is an object to provide a new electron detector ofsimple construction which selectively detects secondary electrons from aspecimen and highenergy reflected electrons therefrom.

It is a further object to provide devices in which the electron detectorof the invention is effectively utilized, and which are arranged toeliminate noise.

Briefly stated, the present invention resides in an electron detectorcomprising an electrically conductive film for detecting secondaryelectrons emitted from a specimen subjected to electron bombardment andan electron collector disposed behind said conductive film, fordetecting high energy reflected electrons reflected from said specimenand passing through said conductive film.

The invention further provides a device in which the electron collectorof the invention is combined with a igh-energy reflected electroncollector, and the outputs of the two collectors are amplified byrespective amplifiers, whose outputs are combined in a mixer, a powersource being provided for elimination of secondary electrons.

The nature, principle, and details of the present invention will be moreclearly apparent by reference to the following detailed description withrespect to preferred embodiments of the invention, when read inconjunction .'ith the accompanying drawing in which like parts aredesignated by like reference characters, and in which:

FIG. 1 is a schematic diagram showing the essential composition andarrangement of elements for detecting secondary electrons;

FIG. 2 is a side view showing one example means for supporting theelectrically conductive film used in the invention;

FIG. 3 is a schematic diagram showing one embodiment of the invention;

FIG. 4 is a block diagram showing another embodiment of the invention.

Referring to FIG. 1, the apparatus shown therein comprises essentially aspecimen 1, an electrically conductive thin film 2, and an indicatinginstrument 3. When an electron beam a is projected onto the specimen 1,an output 12 consisting of a combination of high-energy refiectedelectrons and secondary electrons is produced. The conductive film 2,which comprises a thin film made of an electrically conductive materialsuch as, for example, aluminum, is disposed in the path of this output5. By suitably selecting beforehand the thickness of the thin film, thehigh-energy reflected electrons c are transmitted, and only thesecondary electrons are detected, and the quantity of electrons socollected is indicated by the indicator 3.

If the energy of the rel ected electrons is assumed to be 20 key. itsrange will be of the order of approximately 3 microns. Therefore, byusing a film thickness of the order of angstroms, it will be possible toeliminate fully reflected electrons in this case.

One example of a method of supporting the conductive film is illustratedin FIG. 2. As shown, a conductive film 4 in the form of, for example, athin film of aluminum, is formed by a method such as evaporationdeposition on the surface of a block 5 made of a non-conductive materialof relatively low electron reflection coefficient and secondary electrongeneration efiiciency such as, for example, a synthetic resin. Theconductive film 4 is provided with a lead 6.

In one example of the invention as shown in FIG. 3, the supportingmember (platform) for an electron collector is formed by thescintillator of a scintillation counter 7. By this arrangement, thesecondary electrons are detected by the electron collector 4, and thehighenergy reflected electrons are detected by the scintillation counter(photomultiplier) 7.

It is to be understood that in the examples illustrated in FIGS. 1, 2,and 3, means may be provided to apply a suitable voltage to the electroncollectors 2 and 4 and to obtain the energy spectrum of the secondaryelectrons.

in another embodiment of the invention as shown by block diagram in FIG.4, means are provided to eliminate noise produced in conductive films.As shown, there are provided the secondary electron collector 4, acollector 8 for high-energy reflected electrons, amplifiers 9 and 16, amixer 11, and an electric power source 12, for elimination of secondaryelectrons.

In the operation of this device, the output signals of the secondaryelectron collector 4 and the high-energy reflected electron collector 8are respectively amplified by the amplifiers it and 9 and then combinedin the mixer 11. During this operation, the intensity of noise producedbecause of absorption of high-energy reflected electrons by the thinfilm with respect to the secondary electron collector 4 is, of course,proportional to the intensity of high-energy reflected electronsdetected by the highenergy reflected electron collector 8.

Therefore, the gains of the amplifiers 9 and 10 are suitably adjusted,and the output of the mixer 11 is caused to be zero when the device isin a state where secondary electrons do not reach the conductive film 4.The power source 12 is provided to supply voltage for the purpose ofpreventing secondary electrons from reaching the conductive film 4 atthe time of gain calibration of the amplifiers. By this composition andarrangement, only the intensity of the secondary electrons reaching theconductive film 4 always appears in the output of the mixer 11.

As described above, the present invention provides the conductive filmof very simple composition and arrangement which selectively detectsonly secondary electrons. Furthermore, :by installing the detector shownin FIG. 3 in an X-ray microanalyzer, two scanning images, namely, ahigh-energy reflected electron image and a secondary electron image, canbe obtained at the same time.

Moreover, by the arrangement indicated in FIG. 4, even when, forexample, the primary electron energy is low, and those reflected whichare absorbed by the secondary electron collector increase, the efiect ofnoise thereof is eliminated. Also, in the case where an electron beam isto scan over a specimen, variations such as the variation of reflectedelectron intensity due to variations of the atomic numbers of theelements constituting the specimen are constantly monitored by thereflected electron collector. Accordingly, noise can be eliminated.

It should be understood, of course, that the foregoing disclosurerelates to only preferred embodiments of the invention and that it isintended to cover all changes and modifications of the examples of theinvention herein chosen for the purposes of the disclosure, which do notconstitute departures from the spirit and scope of the invention as setforth in the appended claims.

What I claim is:

1. An electron detector comprising an electrically conductive film fordetecting secondary electrons from a specimen subjected to electronbombardment and an electron collector disposed behind said conductivefilm, for detecting high-energy reflected electrons reflected from saidspecimen and passing through said conductive film.

2. The electron detector according to claim 1, wherein said electroncollector comprises a photomultiplier including a scintillator.

3. The electron detector according to claim 1, wherein said detectorfurther comprises a first amplifier for amplifying the output of saidconductive film, a second amplifier for amplifying the output of saidcollector, an electric power source for preventing the secondaryelectrons from reaching said conductive film, a switch inserted betweensaid conductive film and said power source, and means for producing aditference between the two outputs of said first and second amplifiers.

References Cited UNITED STATES PATENTS 2,814,730 11/1957 Fechter 250-4952,958,779 11/1960 Spear 250-715 3,165,629 1/1965 Oakbe 250-49.53,239,664 3/1966 Farrell 250--49.5

RALPH G. NILSON, Primary Examiner.

A. L. BIRCH, Assistant Examiner.

